This invention relates to a method of and apparatus for overwriting information on a magneto-optic disk.
Due to their ruggedness, removability, and extremely large capacity, optical disks are highly attractive as information storage media. So-called write-once optical disks are used for archival storage of document and image information. Magneto-optic disks offer the further advantage of erasability, enabling their contents to be modified by erasing old information and writing new information in its place. For maximum speed, a magneto-optic disk should be directly overwritable; that is, it should be possible to erase information in a track or sector and write new information in the same track or sector during a single revolution of the disk. It is also desirable that the overwriting be accomplished by modulating a beam of light rather than by modulating a magnetic field, since a light beam can be modulated at high frequencies without producing troublesome electromagnetic noise.
A type of magneto-optic disk that permits direct overwriting by a modulated light beam comprises two magnetic layers: a storage layer and a reference layer. Bits of information are stored as the direction of magnetization of domains called bit cells in the storage layer. The reference layer normally has the same magnetic orientation as the storage layer, but also has the property that its direction of magnetization reverses at high temperatures. The magnetic characteristics of the two layers are controlled so that at room temperature the magnetic orientation of the storage layer is transferred to the reference layer, but at a higher temperature the magnetic orientation of the reference layer is transferred to the storage layer.
A prior-art method for overwriting bits on such a magneto-optic disk is to illuminate their bit cells with a single light beam as the disk spins. When a bit cell enters the beam spot it is momentarily heated. As its temperature rises, first the net magnetic orientation in the reference layer reverses, then the reversed orientation is transferred to the storage layer. After the bit cell leaves the beam spot and its temperature falls, the storage layer retains its new, reversed orientation. As the temperature approaches room temperature the reference layer first reverses to its old orientation, but then acquires the new orientation of the storage layer.
To write new information by this method, first the old information on the disk is read and compared with the new information to find those bit cells in which the new value differs from the old. The beam is switched on for those bit cells, and switched off for bit cells in which the old and new values are the same.
One problem with this method is that the need to compare the old information with the new information and modulate the beam according to whether they are different or the same unnecessarily complicates the overwriting process and its control logic. It would be simpler to modulate the beam according to the new information alone.
Another problem is that the beam must be switched on precisely over the bits to be changed, and switched off precisely over the bits that remain the same. Any beam positioning error may result in illumination of the wrong bits, hence in writing of incorrect information. The minimum length of a bit cell on a magneto-optic disk is currently shorter than one micrometer, e.g., about 0.5 micrometers, so the beam illumination must be controlled with a precision considerably exceeding one micrometer. Equipment capable of such high positioning accuracy is difficult and expensive to manufacture.